As a detector for a gas chromatograph, various types of detectors have been practically applied, such as a thermal conductivity detector (TCD), electron capture detector (ECD), flame ionization detector (FID), flame photometric detector (FPD), and flame thermionic detector (FTD). Among these detectors, the FID is most widely used, particularly for the purpose of detecting organic substances. The FID is a device that ionizes sample components in a sample gas by hydrogen flame and detects the resultant ion current. It can attain a wide dynamic range of approximately six orders of magnitude. However, the FID has the following drawbacks: (1) Its ionization efficiency is low, so that its minimum detectable amount is not sufficiently low. (2) Its ionization efficiency for alcohols, aromatic substances, and chlorine substances is low. (3) It requires hydrogen, which is a highly hazardous substance; therefore, an explosion-proof apparatus or similar kind of special equipment must be provided, which makes the entire system more difficult to operate.
On the other hand, as a detector capable of high-sensitivity detection of various compounds from inorganic substances to low-boiling organic compounds, a pulsed discharge detector (PDD) has conventionally been known (for example, refer to Patent Document 1). In the PDD, the molecules of helium or another substance are excited by a high-voltage pulsed discharge. When those molecules return from their excited state to the ground state, they generate optical energy. This optical energy is utilized to ionize a molecule to be analyzed, and an ion current produced by the generated ions is detected to obtain a detection signal corresponding to the amount (concentration) of the molecule to be analyzed.
In most cases, the PDD can attain higher ionization efficiencies than the FID. For example, the ionization efficiency of the FID for propane is no higher than 0.0005%, whereas the PDD can achieve a level as high as 0.07%. Despite this advantage, the dynamic range of the PDD is not as wide as that of the FID; the fact is that the former is lower than the latter by one or more orders of magnitude. This is one of the reasons why the PDD is not as widely used as the FID.
The most probable constraining factors for the dynamic range of the conventional PDD are the unstableness of the plasma created for the ionization and the periodic fluctuation of the plasma state. To solve this problem, a discharge ionization current detector has been proposed (for example, refer to Patent Document 2). This detector uses a low-frequency AC-excited dielectric barrier discharge (which is hereinafter referred to as the “low-frequency barrier discharge”) to create a stable and steady state of plasma. The plasma created by the low-frequency barrier discharge is non-equilibrium atmospheric pressure plasma, which does not become hot as easily as the plasma created by the radio-frequency discharge. Furthermore, the periodic fluctuation of the plasma, which occurs due to the transition of the voltage application state if the plasma is created by the pulsed high-voltage excitation, is prevented, so that a stable and steady state of plasma can be easily obtained. Based on these findings, the present inventors have conducted various kinds of research on the discharge ionization current detector using a low-frequency barrier discharge and have made many proposals on this technique (for example, refer to Patent Documents 3 and 4).
As described previously, the low-frequency barrier discharge creates a stable plasma state and is also advantageous for noise reduction. Therefore, the discharge ionization current detector using the low-frequency barrier discharge can attain a high SN ratio. However, the conventional discharge ionization current detector using the low-frequency barrier discharge also has problems one of which is poor linearity of detection sensitivity.
FIG. 3 is a graph showing an example of actually measured values of the detection sensitivity in the FID and a conventional general-type discharge ionization current detector when octane (C8H18) is measured. In FIG. 3, the horizontal axis indicates a logarithmic value of a sample introduction amount, while the vertical axes indicate the detection sensitivity. In the vertical axes, the left-hand axis indicates a scale for the FID while the right-hand axis indicates a scale for the discharge ionization current detector. As for the discharge ionization current detector, it can be recognized that the absolute value of the detection sensitivity is higher than that for the FID by approximately two orders of magnitude, while the range where the linearity of the sensitivity with respect to the sample introduction amount is maintained is narrower than that for the FID. The sensitivity remarkably decreases particularly in an area where the sample concentration is high. Although not shown in FIG. 3, the linearity of the sensitivity is practically kept in the area lower than 0.01 ng for the FID. In the FID, the linearity of the sensitivity is kept over a range of approximately seven orders of magnitude of the sample introduction amount. On the other hand, the linearity of the sensitivity is kept over a range no wider than four orders of magnitude of the sample introduction amount for the discharge ionization current detector.